The present invention relates to an arrangement for the sealing-off of metallic cylindrical bodies, particularly shafts in cryogenic valves, the shaft being arranged such that it can be slid coaxially in the axial direction in a cylindrical housing, and being surrounded by an essentially ring-shaped sealing device made of a material that cannot be glued to metal.
In the case of cryogenic valves, as they are required particularly for space missions, the requirement exists that sealing devices, particularly plastic--metal connections, must be stable in the temperature range of 20K&lt;T&lt;350K for many years; that is, that no leakages must occur which negatively influence the sealing effect. Such leakages may occur particularly as the result of different shrinkages of metal and plastic in the case of major temperature changes. A plastic sealing device which is placed in a press fit on a shaft to be sealed off and which consists of a material such as pure teflon or Vespel, that is, materials which cannot be glued to metals, depends in its sealing effect on the ambient temperature; that is, that in the case of different amounts of shrinkage of plastic and metal an only insufficient sealing is achieved at lower temperatures.
For several years, metal alloys have been known which have shape memory characteristics and which have the capability of abruptly changing their dimensions in the case of a predetermined temperature. Several of these alloys have a one-way characteristic; that is, when the transition temperature is exceeded, they maintain the resulting changed size or dimension. In contrast, other alloys have a reversible, that is, a two-way characteristic, in which case, when there is an exceeding of or falling below the transition temperature, a defined dimensional change is obtained (Zeitschrift fur wirtschaftliche Fertigung (Journal for Economical Manufacturing), Volume 12, 1986, Pages 703, et. seq.).
The firm Raychem Corporation in Menlo Park, Calif., U.S.A., offers electric couplers (Brochure M1928, H 50772, July 1984) in which a sleeve consisting of a metal alloy with shape memory characteristics and interior sealing cutting edges surrounds the end pieces of two abutting pipes that are to be connected and consist, for example, of steel, in which case the holding force which is generated by the shrinking of the sleeve considerably exceeds that of a welding.
It is an object of the present invention to provide an arrangement for the sealing-off of shafts in cryogenic valves which ensures a sealing effect over a period of several years which is independent of temperatures, while the metal alloys with shape memory characteristics are used which are known per se, so that within the temperature range of from 20K to approximately 350K, a high prestress is maintained within the arrangement and a sliding-off of the sealing device during axial movements of the shaft is reliably avoided.
Based on an arrangement of the initially specified type, it is suggested for achieving this object that the sealing device be surrounded by a ring which is arranged concentrically with respect to the shaft which is slidable coaxially in a cylindrical housing, consists of a metal alloy with shape memory characteristics and with a one-way characteristic and whose interior shape is adapted to the exterior shape of the sealing device and whose crystal structure is arranged such that, when the transition temperature is exceeded, the ring contracts radially and as a result connects the sealing device in a force-locking and form-locking manner with the housing.
Advantageously, the ring-shaped sealing device is provided with a groove which extends coaxially with respect to its opening and in which a spring is inserted which spreads the groove radially. In this case, the spring may consist of a band which is arranged in the groove bottom and which is provided with spring shackles arranged on both sides which extend at a distance from one another along the groove walls.
Preferably, the ring surrounding the sealing device has an essentially L-shaped cross-section, in which case that interior wall of the ring which rests on the exterior wall of the sealing device in parallel to the exterior wall of the shaft is provided with several sealing cutting edges.
The arrangement of the housing may have a flange which surrounds the shaft concentrically and which has a collar which projects concentrically upward to the shaft and which engages into a fitted recess in the sealing device.
The exterior wall of the collar is advantageously provided with at least one sealing cutting edge which, in an axial sectional view, is arranged between two adjacent sealing cutting edges on the interior wall of the ring.
Instead of the arrangement of the housing having a flange, the ring and sealing device may also be connected directly with a housing outer wall.
The arrangement according to the invention has a simple construction and offers a constant sealing effect over the whole temperature range of 20K&lt;T&lt;350K while the desired high prestress is maintained. It also offers the advantage of cost-effective manufacturing and can be arranged with few manipulations on the shaft that is to be sealed off correspondingly. A heating above the transition temperature of the material with the shape memory characteristics and with the one-way characteristic has the effect that the ring contracts abruptly, and the sealing device made of plastic which otherwise cannot be connected with metal and which consists, for example, of pure Teflon, of PCTFE, FEP, PEEK, Vespel or other high-temperature thermoplastics, is pressed with enormous force on the exterior wall of the cylindrical housing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.